This invention relates to improvements in methods and apparatus for freezing foods and similar articles employing I.Q.F. (individually quick-frozen) techniques wherein the articles in a "fluidized" bed are frozen in an individually separate state rather than in a cohesive state. More particularly, the invention is directed to improvements that reduce the installation and operating costs of such systems while producing a frozen product of superior quality.
The term "fluidized" in the present context refers to the condition in which a mass of solid articles tends to act somewhat as a fluid, that is, a condition wherein the articles are semi-suspended individually and relatively motile somewhat like the molecules of a fluid. In I.Q.F. food processing, fluidization is most commonly created or maintained by forced flow of a gas such as air passed upwardly through the mass.
It was long ago discovered that food articles such as beans, peas, berries, cut corn, etc. could be quickly and individually frozen when subjected to a fluidized bed condition created by an updraft of air or other gas cooled to subfreezing temperatures. Due to the tendency of the articles to stick together because of moisture (i.e., wash water and/or juices), reliably achieving and maintaining fluidization required not only a thin bed of articles but a bed of uniform thickness and uniformity of air flow through the bed. In practice these conditions were difficult to achieve, leading to development of various means of "excitation", assisting the mass of articles to enter and maintain a state of fluidization. Some of these excitation techniques included use of secondary air jets at the entrance end of the foraminous conveyor supporting the articles so as to induce augmented vertical motion of the articles at that location where they are most wet and sticky. A "reload" bar of half-oval configuration placed transversely across and immediately above the conveyor belt at a point down the line of conveyance was also used. It served to dislodge any articles that might begin freezing to the conveyor belt, functioned as a leveling weir which helped redistribute the fluidized mass of articles more uniformly across the full width of the conveyor belt and helped insure maintenance of fluidization as freezing continued. Those and other excitation devices were also considered necessary to permit safely increasing belt loading depth in order to satisfy increasing commercial production capacity demands on prior systems.
With the ability to freeze the articles in a fluidized bed of increased depth, it became feasible to run the conveyor belt more slowly and thus to produce improved freezing action even with the system operating at high evaporator temperatures so as to minimize energy consumption by the compressors. However, for practical purposes the fluidized bed depth in such prior single-conveyor freezing systems was limited to about 2 inches when operating with initially wet vegetables or fruits since with greater depths it became excessively difficult to induce and maintain fluidization by any means.
By operating two independent fluidized bed freezing systems in series, with the conveyor of the second system running at a materially lower speed than the conveyor in the first system, it became possible in one prior art installation to load the second conveyor with vegetables such as corn to a depth of 5 to 6 inches. This permitted operating the tandem systems at still higher evaporator temperature, hence, more efficiently, without impairing the uniformity of deep freeze imparted to the final product. In fact, it permitted freezing the product colder than in earlier systems with less expenditure of energy and without unduly increasing the plant floor area occupied by the overall system. In that endeavor, cut corn partially frozen on the conveyor of the first system, for example, was discharged through a connecting air conveyor onto the more slowly moving conveyor of the second system which completed freezing of the product down to the desired final temperature. A higher quality product resulted.
Despite the advantages accruing from the tandem dual freezer system installation just described, I.Q.F. operation in both freezers required the usual critical attention to creating and maintaining full fluidization conditions in both systems. This implied observance of full system design and operating criteria in the first system as well as the second, and required continued attention to product loading uniformity and depth, and to air flow uniformity and vigor.
As a reult of recognizing these continued shortcomings but also the advantages of the dual tandem system just described, it became apparent in the conception of the present invention that an improved two-stage system was possible that could take best advantage of a short-run deep bed noncritical fluidization second stage freezer operated at higher, more efficient evaporator temperature, by combining it with a simplified, noncritical article-conditioning first stage. More specifically, it was discovered that preparing the articles for introduction to the deep bed fluidization second stage in a condition compatible with easily maintained deep bed fluidization freezing there could be accomplished without the requirement of a critically designed and critically operated fluidized bed first stage article freezer.
In the latter regard, it was also recognized that the older concept of precooling food articles for a short time in refrigerated air held at or about freezing temperature (i.e. approximately 30.degree.F.) before entering the freezer itself would be ineffective conditioning preliminary to deep bed fluidization freezing employing bed depths in the range of 4 to 6 inches, for example. This kind of conditioning removed some of the sensible heat but left the articles at a temperature well above freezing with little affect upon their sticking tendency due to surface moisture. That possibility was therefore quickly rejected as offering no answer to achieving the objectives herein.
In accordance with this invention, a two-stage system is provided which minimizes compressor energy consumption, reduces equipment costs, minimizes plant floor area requirements, and yields frozen products of still better quality. A more specific object is to devise a deep bed fluidization freezer combined with an effective but versatile article-conditioning first stage, relatively noncritical of operation and relatively noncritical of design, capable of feeding articles to the deep bed fluidization stage in a superior condition for the purpose, compared with earlier precoolers and earlier first stage product freezers.